"This is a nice filter, if used properly. Extremely easy to assemble, this filter will be best in small tanks (10g MAX), such as fry tanks. It poses no risk to young fish, all it does is pump ait through a tube, dispacing water through the filter system. It holds a sponge that can easily support the bacteria nessecary for the nitrogen cycle. The sponge should get dirty in a few days, but that's good. That means there IS the good bacteria.
Also, it's kind of difficult, but you can take apart the part with the sponge and clean the spnge out. All I suggest is ONE sqeeze under warm water, or else you'll have to cycle your tank all over again. Carefully put the sponge back in the cartridge, and put the top over it. Sometimes it stays on, sometimes it doesn't. If not, just tie it on with some thread. The only time you'll ever need to replace these cartidges is about twice a year, or if you get any serious diseases you'll want to start over.
Overall, this filter is very good for small tanks, like I said, but if your tank is bigger than 10 gallons, don't bother."